The use of pharmaceuticals (e.g., as diagnostic imaging agents, therapeutic agents, etc.) to detect and treat cancer is well known. In more recent years, the discovery of site-directed radiopharmaceuticals for cancer detection and/or treatment has gained popularity and continues to grow as the medical profession better appreciates the specificity, efficacy and utility of such compounds.
These newer radiopharmaceutical agents typically consist of a targeting agent connected to a metal chelator, which can be chelated to (e.g., complexed with) a diagnostic metal radionuclide such as, for example, technetium or indium, or a therapeutic metal radionuclide such as, for example, lutetium, yttrium, or rhenium. The role of the metal chelator is to hold (i.e., chelate) the metal radionuclide as the radiopharmaceutical agent is delivered to the desired site. A metal chelator which does not bind strongly to the metal radionuclide would render the radiopharmaceutical agent ineffective for its desired use since the metal radionuclide would therefore not reach its desired site. Thus, further research and development led to the discovery of metal chelators, such as that reported in U.S. Pat. No. 5,662,885 to Pollak et. al., hereby incorporated by reference, which exhibited strong binding affinity for metal radionuclides and the ability to conjugate with the targeting agent. Subsequently, the concept of using a “spacer” to create a physical separation between the metal chelator and the targeting agent was further introduced, for example in U.S. Pat. No. 5,976,495 to Pollak et. al., hereby incorporated by reference.
The role of the targeting agent, by virtue of its affinity for certain binding sites, is to direct the diagnostic or therapeutic agent, such as, for example, a radiopharmaceutical agent containing the metal radionuclide, to the desired site for detection or treatment. Typically, the targeting agent may include a protein, a peptide, or other macromolecule which exhibits a specific affinity for a given receptor. Other known targeting agents include monoclonal antibodies (MAbs), antibody fragments (Fab and (Fab)2), and receptor-avid peptides. Donald J. Buchsbaum, “Cancer Therapy with Radiolabeled Antibodies; Pharmacokinetics of Antibodies and Their Radiolabels; Experimental Radioimmunotherapy and Methods to Increase Therapeutic Efficacy,” CRC Press, Boca Raton, Chapter 10, pp. 115-140, (1995); Fischman, et al. “A Ticket to Ride: Peptide Radiopharmaceuticals,” The Journal of Nuclear Medicine, vol. 34, No. 12, (December 1993):
In designing an effective compound for use as a diagnostic or therapeutic agent for cancer, it is important that the drug have appropriate in vivo targeting and pharmacokinetic properties. For example, it is preferable that for a radiopharmaceutical, the radiolabeled peptide has high specific uptake by the targeted cells. In addition, it is also preferred that once the radionuclide localizes at a targeted site, it remains there for a desired amount of time to deliver a highly localized radiation dose to the site.
Moreover, developing radiolabeled peptides that are cleared efficiently from normal tissues is also an important factor for radiopharmaceutical agents. When biomolecules (e.g., MAb, Fab or peptides) labeled with metallic radionuclides (via a chelate conjugation), are administered to an animal such as a human, a large percentage of the metallic radionuclide (in some chemical form) can become “trapped” in either the kidney or liver parenchyma (i.e., is not excreted into the urine or bile). Duncan et al.; Indium-111-Diethylenetriaminepentaacetic Acid-Octreotide Is Delivered in Vivo to Pancreatic, Tumor Cell, Renal, and Hepatocyte Lysosomes, Cancer Research 57, pp. 659-671, (Feb. 15, 1997). For the smaller radiolabeled biomolecules (i.e., peptides or Fab), the major route of clearance of activity is through the kidneys which can also retain high levels of the radioactive metal (i.e., normally >10-15% of the injected dose). Retention of metal radionuclides in the kidney or liver is undesirable.
Conversely, clearance of a diagnostic or therapeutic agent from the blood stream too quickly by the kidney is also undesirable if longer diagnostic imaging or high tumor uptake for radiotherapy is needed. The retention of the radiopharmaceutical compound in the patient is often measured in terms of half life (i.e., the time it takes for one half of the administered dosage to be cleared from the patient). A radiopharmaceutical compound with a shorter half life indicates that it is cleared from the patient at a faster rate than a radiopharmaceutical compound with a longer half life.
A new and improved method for improving the half life of pharmaceutical compounds has now been found leading to improved diagnostic and therapeutic agents.